


Happy Birthday Sans!

by absolutedepths



Category: Undertale (Video Game)
Genre: Choking, Collars, M/M, Pain, Plot, Sensitive bones, Spitroast, selfcest, sub!sans
Language: English
Status: Completed
Published: 2015-12-28
Updated: 2015-12-28
Packaged: 2018-05-09 23:41:42
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,633
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/5560372
Author URL: https://archiveofourown.org/users/absolutedepths/pseuds/absolutedepths
Summary: <blockquote class="userstuff">
              <p>It's Sans' birthday, and the two robots want to do something nice for the skelly.</p>
            </blockquote>





	Happy Birthday Sans!

 

Sans had found himself with a lot more energy since they had left the underground. His days were spent mostly in Alphys' lab, knee deep in science projects, sharing observations with Alphys and working on his own side projects. He had a passing interest in robotics, his main field of interest these days being quantum physics. Alphys appreciated having Sans in the lab with her, usually only sharing the space with Neo, who had begun living with her a few years prior after she built him for company. Undyne sometimes dropped in, but the science nerds were usually wrapped up in a project, so Undyne waited until after work hours to hang out with Alphys.

 

Sans quickly grew used to Neo's presence in the lab, though initially he hadn't been comfortable with the tall robot looming over him. He looked and sounded like Mettaton in many ways, and his form reminded Sans of a time long past. As time went on though, Neo's personality grew very separate to Mettaton's, and his ever present curiosity meant Sans and Neo often chatted over work. Neo was a freeloader for the most part, being one of Alphys' creations, but he did his part helping out with Sans and Alphys.

 

Mettaton was another regular in the lab, though when he was around the conversation always turned to sex. The skeleton quickly learned that Mettaton was curious about everyone's sex lives, to a fault, but indulged him in idle chatter about it anyway. Often, when Alphys was busy working or watching a series finale, Sans stepped in to do repairs on the two robots. Mettaton made it into a game – teasing Sans with moans or lewd comments as he inspected his exterior. Mettaton's cocky nature was toned down whenever Sans was inspecting Mettaton's insides, though, the robot choosing to hum to himself or talk about something completely different. Was it embarassment? Sans had no idea, but it was a nice break.

 

One particularly harsh winter on the surface, Sans' birthday had rolled around. He spent most of the day at home with Papyrus, but dropped into the lab in the evening to get some work done. His work was flowing incredibly well lately, and he wasn't about to let his birthday get in the way of progress. To his surprise, when he entered the lab Mettaton and Neo were waiting for him in the usual work area. Mettaton was sitting on a bench, swinging his legs. Neo was sitting at Alphys' usual spot, using her computer to watch some anime. Both robots turned their heads as Sans made his way inside, dropping his shoulderbag and chuckling to himself.

 

"It's a bit late for you two to be around, isn't it?" Sans continued walking over to his desk, taking a seat in his office chair and turning to Mettaton. He had to look up to make eye contact with the robot who was smiling down at him. "You've _bot_ to be kidding me, you guys didn't break something again, did you?" Mettaton looked legitimately confused. "I meant, you guys didn't break anything on _each other._ " 

 

From across the room, Neo dropped the figure he was toying with on the ground and squeaked. Mettaton simply narrowed his eyes and put on a cocky smile. "That's an interesting accusation, darling."

 

Sans shrugged. "You guys really underestimate how often I come to the lab after dark. Neopet, huh? That's a cute name." Sans purposely directed his attention back to the computer in front of him.

 

Neo shot up from the chair he was on and yelped "Mettaton!!" at his partner, blushing profusely. Sans chuckled to himself as Mettaton shrugged at Neo.

 

"You're pretty vocal, Neo."

 

Neo didn't know how to respond, so he went back to watching anime. Sans didn't turn to look, but he suspected the taller of the two robots was sulking. Mettaton laid down on the bench he was on, bringing his face closer to Sans. "Darling, I have to ask, why are you here on your birthday?"

 

Sans paused his typing to glance up at the glam-bot. "How did you know it was my birthday?" Mettaton chuckled in response.

 

"You told us about your birthday last year, darling." Sans' eye sockets widened. _He remembered something I mentioned a year ago?_ Mettaton read his reaction and answered without prompt. "I never forget the birthday of a fan." 

 

Sans chuckled. "I'm not your fan, Metta." Mettaton took his turn to laugh, a cocky smile over his vinyl lips.

 

"Really, Sansy? I consider most people I meet to be my fans." He leant forward further, his voice dropping into a purr. "Especially those who look at me with lusty eyes."

 

A light blue tint of blush swept across Sans' cheeks, but his poker face did not betray him further. The skeleton leant forward and tapped the bridge of Mettaton's nose. "Keep dreamin', kid."

 

"Sans, you're blushing." Mettaton chided, moving to sit upright on the bench. Sans' hands clapped to his cheekbones and he through an angry glare in the glambots direction, gritting his teeth.

 

"Boy howdy, isn't it hot in here, Neo?" Sans called out, louder than he needed to considering Neo wasn't too far away from them. Neo glanced at Sans, then to the air conditioner which was pumping out cool air, then back to the two over at Sans' desk. He blinked at them, staring blankly.

 

"...No?"

 

Sans groaned and Mettaton laughed, loudly and without grace. Neo continued to stare at Sans blankly, his brow slightly furrowed as he tried to work out what the joke was. After a minute or so of that Mettaton had quitened down, so Sans returned his gaze to him. "Look kiddos, I haven't gotten a skele-ton of work done tonight, so I'll be getting back to that now. No distractions, I mean." Neo shrugged his shoulders as if to say 'not my problem', but the other robot in the room didn't seem satisfied with that. He returned to lounging across the desk like a chaise, gazing at the skeleton with lusty eyes.

 

"Saaaaaaaaans~"

 

"...Yes, Mettaton?"

 

"Remember when you told me about your kinks?"

 

Sans grit his teeth and continued to type, pulling out past files from the computer's database. "Yes, I do."

 

"Remember how it's your birthday?"

 

Sans stole a glance in Mettaton's direction, looking at the robot practically hanging off the desk. "I do, in fact, remember that."

 

"Did you get any good presents this year?"

 

Sans rolled his eyes. "Yeah, I did, Paps and Frisk came over earlier to give them to me."

 

"Would you like another one?"

 

Sans gave up typing and clicking, turning in Mettaton's direction again, pushing back the wheeled chair from the desk. He crossed one leg over the other and sighed. "Where are you going with this?"

 

Mettaton looked pleased with the attention back on him and pulled himself up from the edge of the desk, sitting with his legs hanging over it once more. "Neo and I were talking, and we agreed, we both think you deserve a break, Sansy." Sans moved to speak, but Mettaton continued, his eyes silently telling him to shut up and wait. "We agreed on a way to do it, too, but only if you agree." His hard stare softened as he waited for an answer. 

 

Sans narrowed his eye sockets, thumb absently pressed on his kneecap as he considered it. "You want to live out my kinks?"

 

"Yes, Sans. Neo and I want to fuck you."

 

The skeleton instinctually shifted eye contact from Mettaton to the computer in front of him. "Not interested, pal."

 

"Your face tells a different story, Sans." Neo called out from across the room.

 

"Why do you choose now to be observant?" Sans hissed back at him, groaning afterwards. Neo frowned in response and crossed his arms.

 

"Come on, Sans. You need to relax a little, unwind, won't you just try it? We'll stop if you don't like it." Mettaton cooed.

 

Sans put his head in hands and sighed. "Alright, fine, we'll try it."

 

Mettaton practically hit the ceiling, launching off the desk and wrapping his noodley arms around the blushing mess of a skeleton beneath him. "Ooooh, Sansy, I knew you'd say yes~! You're going to have a  _great_ time, I swear~!" As quickly as he had rushed over, Mettaton hopped back, face serious with his gaze turned to Sans. "We have to talk, first, okay? You like it a little rough, honey. We need a safe-word." 

 

Sans grinned. "How about short circuit?"

 

Mettaton gave him a pleased smile in return. "You have to be sure to remember it, okay?" Sans nodded back at him.

 

"Sure thing."

 

Mettaton clapped his hands together and his grin widened. "Well darling, there's no time like the present." The titular robot stepped forward, placing his hands on the office chair either side of Sans' head. The chair had no arm rests, so he sat down on the skeleton's lap, straddling his legs. Sans knew Mettaton's legs were soft, but had never felt them like this before. Mettaton enjoyed the lusty look Sans was giving him, wiggling his ass a bit to show off.

 

Sans didn't have time to linger on the feeling of Mettaton's ass because a hand was already snaking its way under his shirt. Mettaton hummed as he pulled up the hem of the fabric, exposing Sans' ribcage. "Are you gonna bone me now?" Sans chuckled, placing his hands on Mettaton's thighs.

 

Mettaton's charming facial expression dropped into a harsh snarl. "Hands by your sides, darling." Sans was shocked but his hands dropped off Mettaton straight away and he felt his cheeks grow hot. Mettaton giggled softly, the tips of his gloved finger running across Sans' bottom rib. The skeleton shuddered in his place and looked down at Mettaton's lap, breaking their eye contact. "You're surprisingly sensitive, has it been a while?"

 

Sans suspected it wasn't an honest question, Mettaton was more interested in hearing Sans answer out loud than what it actually was. Considering that, he chose not to answer. Mettaton must have noticed this, choosing to change his light touch to something a little firmer, hooking over Sans' second last rib and squeezing gently. Sans let out a choked breath.

 

"Oh my god, Sans. I haven't even gotten to fucking you yet and you're so sensitive." Mettaton sounded impatient. "You're making this too easy for me." Sans gave him a glare that melted into a blue tinted blushing frown as Mettaton's other hand ghosted just over his spine.

 

"Metta, you're really getting under my skin..."

 

"Sans, you don't have skin."

 

"...Yeah I sorta realised how dumb that sounded as soon as I said it."

 

Mettaton pressed a single finger to Sans' mouth, shushing him gently. The boy glanced up at Mettaton as he did this, eyes meeting with Mettaton's confident gaze. Mettaton leant forward softly, motioning for a kiss, so Sans instinctually closed his eyes. A ghost of breath fell over Sans' mouth and he heard a click around his neck. His eyes fluttered open to Mettaton holding what looked like a strap of leather inbetween his teeth, waggling his eyebrows provocatively. The skeletons hands shot to his neck where they softly fingered leather and metal.

 

"A collar?" Sans choked out.

 

Mettaton took the lead out of his mouth, holding it in his right hand. "That's right, Sansy." His cocky demeanor dropped for just a second. "You don't mind, do you...?" Sans shook his head violently in reply. Mettaton's grin returned and he returned to his previous attitude, shuffling off the smaller's lap and standing about a metre ahead of him. His eyes danced along the lead to Sans' neck and his eyes sparkled with curiosity. Holding back an excited squeal, Mettaton yanked hard on the lead, pulling Sans forward by the neck. He tumbled off the chair and onto the floor on his hands and knees. Mettaton pushed the shit eating grin off his face and walked around behind Sans, placing his heel on the skeleton's spine. Sans felt the collar slip around so the lead lead off from the back of his neck, before Mettaton pulled, hard. He felt a shudder run through his body as he was pulled backwards, he let out a few heavy pants, face flushed blue.

 

Mettaton seemed pleased with this, feeling his heart grow warmer in its case. He pressed the heel of his boot into the base of Sans' spine, grinding roughly into his bone. Sans yelped and tightened his grip on the collar around his throat, not attempting to move it but clinging to it instead. Mettaton loosened the lead and he fell forward, panting for a few seconds. As soon as he had stopped, Mettaton pulled back again, watching the skelly writhe underneath him panting for air. Mettaton's attention flickered over to Neo who was watching from the corner, face flushed. Mettaton cocked an eyebrow and called out to him. "Darling, you've been awfully quiet. Won't you be a dear and come help?"

 

Mettaton's soft pet names beckoned Neo over, who's heart was already glowing pink and warm. Ex glanced down at his heart and then back up to his face, Neo gazing at Sans with heart filled eyes as he choked against the leather and studs. The taller of the two glambots didn't realise his hand was ghosting over his heart just gently, fingertips dancing over the surface of the gelatin substance. Mettaton held back a chuckle at Neo's eagerness and, pulling Sans' lead so he was on his knees pulled back towards Mettaton's thighs, shot Neo a look. "Remember what we discussed earlier?"

 

"Yeah, okay." Neo mumbled, taking San's hands and guiding him down to the ground. Neo took the lead from Mettaton and fumbled with it as he looped it around his right hand a few times. Neo's cannon arm rested on Sans' shoulder. Another quick glance up to Mettaton, who was beaming with delight and almost pride, and Neo nodded to himself. From the bottom of his torso, mechanical whirrs were heard, and then a panel in the front of crotch opened. Sans wasn't able to chuckle at the amusing display through the choke hold around his neck, but he gave Neo an incredulous look.

 

Neo noticed it and sneered down at him, doing his best to push on a dominant persona. A pink robo-dick emerged from the panel, glittering in the dim light of the lab. Sans snorted and Neo looked disappointed at his reaction, looking up at Mettaton with a desperate facial expression. Mettaton put on a shit eating grin as he gently pressed on Sans' spine for emphasis. "He won't be able to laugh when his mouth is full." Sans shuddered at the subtle threat, looking up at Neo. The way the taller towered above him made his heart throb in his chest. "Open wide, Sansy~" Ex teased from behind him. Sans did as he was told, the pink glittering cock sliding over the top of his outstretched tongue. Neo close his eyes as the cool, slick feeling of Sans' tongue ran up and down him. Sans took it in slowly, gazing up at Neo.

 

Once Neo got comfortable he held Sans' head with his right hand, the lead wrapped around his wrist. His fingertips danced across his skull as if looking for a hold, but Sans' smooth cranium had nowhere decent to grab him. Taking note of this, Neo tightened the lead and pulled backwards, pulling Sans forward by the collar. Neo slid down to the back of Sans' mouth in one smooth motion and the skeleton gagged, choking around the sudden intrusion. Neo let out a pleased moan as Sans' mouth tightened around him, keeping him down to the hilt for a few seconds before allowing him to breathe. Having had his gag reflex toyed with, Sans had a few blue tears rolling down his cheeks. Neo absolutely drank in the view, pulling him forward and roughly thrusting into him.

 

From behind, Mettaton ran a hand down Sans' spine, causing him to shudder roughly and moan around Neo's glitter-dick. Ex took this as encouragement, firmly feeling his spine through his shirt. Mettaton pushed up the fabric to reveal the stretch of spine between pelvis and ribcage and ran his finger along the inside of it. Sans flinched, moving into his touch. Mettaton hummed softly and ran his finger along the hem on Sans' shorts, making sure to brush against his pelvis and hipbones.

 

The light touches from Mettaton and rough mouthfucking were sending Sans insane. His jaw hung almost slack as Neo ground against him, using him like a fuck toy. Neo was shy at first but once he felt confident in a situation, he made an impressive dom. Mettaton finally hooked Sans' shorts and yanked them off him, revealing Sans' bare pelvis and legs, but also something else interesting.

 

It was made of the same ectoplasm matter Sans' tongue was made of, and though Mettaton hadn't touched it yet he assumed it would be warmer than Sans' tongue. It was a dick, the tip sparkling, and above that, a pocket of mock-flesh. Mettaton licked his lips as he leant forward, observing how the dick and...well he supposed it was like Sans' ass? Vagina? Skeleton reproduction was a foreign concept to Mettaton. He ran a finger over the underside of Sans' dick and felt him freeze up in response before shaking his pelvis towards Mettaton. The glambot licked his dry lips and looked up to his double. "Neo, I think I have a present for you." Neo glanced up, his face lightly flushed and biting his lip.

 

"What is it- ohhhh..." he mumbled, following Mettaton's fingers with his eye. "Let's switch."He pulled out of Sans' mouth without warning, leaving the other slack jawed and gasping for air. Neo took Sans by the collar and pulled him forward, having him step out of his shorts and leaving everything under the belt exposed. Mettaton took the opportunity to slip Sans' jacket off him so he was left with only an oversized white tshirt, the thin fabric barely hiding Sans' beating heart underneath. Mettaton walked over to a bench that was just at Sans' chest level, sitting on top of it and spreading his legs out. Neo lead Sans over and Mettaton wrapped his legs around his neck, the heel of one of his boots resting on Sans' shoulderblade.

 

Neo walked around behind Sans, lining his glitter-dick up with Sans' hole. Without warning, he thrust in roughly making Sans weak at the knees. Sans gripped the bench Mettaton sat on for support, leaning his weight into it and letting out a very vocal groan. His voice was higher pitched than usual and both robots thought it made him sound a lot cuter. Mettaton pulled Sans closer to him with his legs and pointed to his heart case with one hand. He hit a small, hidden button on his hip and the case hissed open, exposing his throbbing pink heart. It was slick with goo and the smell of artificial strawberry filled Sans' nose, usually Sans would have found the smell to be sickly sweet, but he began to salivate, leaning forward and opening his mouth to reveal his slick tongue.

 

"Oh, Sansy, you're such a cute little thing." Mettaton placed one hand on Sans' head as he leaned in, the other supporting him behind his back. His thumb stroked the smooth bone as he grinned, Sans' tongue diving into the thick gelatinous matter of his heart. Mettaton let out a moan he disguised as a chuckle and pressed the heel of his boot into Sans' spine. The other groaned at the pain shooting down his spine, shooting Mettaton a goofy, flustered grin.

 

Mettaton absolutely loved the feeling of power he got from having Sans between his legs, his right leg pulling Sans closer to him by the neck and his left digging into his spine. Sans' skull was pushed up against Mettaton's front, the skeleton writhing underneath the pressure on his spine. Neo meanwhile was having the time of his life, running his nails down Sans' back and pelvis, each thrust pushing Sans into Mettaton, making Mettaton shiver. Sans' head spun and all he could feel was the pleasure from Neo and the striking pain of Mettaton, he was losing all tact with his tongue and simply pressing into Mettaton. Neo exchanged a glance with Mettaton, then dug his nails into Sans' spine. The impressive pressure from his sharp nails caused Sans to cry out, almost biting his tongue.

 

"Ah-ah, shit..." Sans choked out, his voice trembling. "N-Neo-please-" Neo's grin widened as Sans called out his name, increasing the pressure in his vice grip around Sans' spine. "Sh-short circuit! Short circuit!"

 

Neo instantly let go, pulling his hand back as if it had been burned. Mettaton also stopped, unwrapping his legs from Sans' neck and shuffling back on the bench to look down at his face. Sans took a few moments to breathe, rib cage heaving and blue soul pulsing underneath his shirt. Neo pulled out from behind Sans, watching to make sure he could still stand, frowning intensely as guilt washed over him. Mettaton was the first to speak. "Are you okay, honey?"

 

Sans was still dizzy, but nodded slowly. Ex glanced up at Neo who had eyes full of concern, he tried to give Neo a reassuring smile, but the other still looked intensely uncomfortable. "'m fine...just got a bit too...too intense there for a second, heh..." he punctuated his sentence with a tired laugh.

 

"I'm sorry, Sans. I'll go." Neo offered, taking a few steps back before turning on his heel and beginning to leave.

 

"Neo! Wait-come back here!" Ex called out, stopping Neo in his tracks. "You don't need to leave, come back over here, okay?" The taller of the two robots gripped his left arm with his right, fingering metal of his arm. "Neo, come on. Sans isn't angry."

 

Neo turned around, eyes to the floor, wandering back over to sit next to Mettaton. Mettaton held Sans' head in his hands, thumbs stroking his cheekbones. "Are you alright?"

 

"Mm, sorry for freaking Neo out." Sans replied, resting his head on Mettaton's soft thighs. Neo seemed to look even more uneasy at that, shuffling slightly and only throwing Sans a quick glance.

 

"Do you want to keep going? We can do something a little more vanilla, if you like." Mettaton offered, leaning over to unclip Sans' collar from his neck. Sans shot Ex a toothy grin.

 

"Yeah, sounds good." He smiled up at Neo too, who seemed to soften a little at that. Mettaton clapped his hands together once, sliding away from Sans and hopping off the bench. He walked across the room and pulled over the wheeled office chair that they had left across the room, sitting down on it and patting his legs.

 

"Hop up."

 

Sans cocked an eyebrow bone as he turned, watching Mettaton's intense gaze. "Uh,..."

 

"Come on, darling."

 

Sans walked over, but as he was about to straddle Mettaton's hips, the robot guided him around with his hands, making Sans sit on his lap so they were both facing Neo. Sans opened his mouth to ask what Mettaton was doing before the robot lifted up his legs with both hands, spreading them apart with his arms hooked under them, exposing Sans completely to the cool air of the lab, and to Neo. A blush swept over Sans' cheeks and he felt his heart/soul beat quicker in his chest.

 

Mettaton's fingertips brushed over the cool bone of Sans' thighs and the smaller leant back into Mettaton, closing his eyes as shivers rained up and down his spine. Mettaton felt boney hands grasp at his thighs for support and chuckled sweetly in his ear. "Now for a surprise, sweetie." Sans' eyes fluttered open as he felt something pressing at his entrance, his head twisting around to look at Mettaton. The two held eye contact as Mettaton thrust upward, Sans' head swimming with the sudden rush of being filled. He grasped at Mettaton's thigh as his eyes fluttered shut and jaw hung slack, letting out a whine. "F-fuck..."

 

"That's what I'm doing." Mettaton chided, and Sans let out a strangled chuckle at his lame joke.

 

Neo watched from across the room, pressing his thighs together as the two worked up a steady rhythm together. He hopped off the desk and sauntered across the room, eyes to the ground. When he reached Sans he was a drooling mess, letting out small whines each time Mettaton thrust into him. Neo leant forward and held one hand to his cheekbone, leaning forward to pull Sans into a kiss. Sans' eyes flittered open as Neo's lips met his teeth, then fluttered shut again as he relaxed into it. Neo laid soft kisses on the skeleton's teeth, humming to himself. Sans' mouth opened up and allowed Neo to pull Sans into a proper kiss, the two of them swept up in each other. Neo's hand held Sans' head softly, his canon arm resting on Sans' shoulder.

 

Neo hooked his fingers around Sans' ribs over the thin fabric of his shirt as the skeleton's cool ectoplasm tongue ran over his synthetic silicone one. Sans whined into Neo's mouth as he drew close, the extra stimulation from his ribs pushing him close very quickly.

 

"You two are so adorable together." Ex beamed, grinding into Sans. "Really, really cute." Sans' blush increased, glowing a vibrant blue. Neo's cheeks were dusted pink too, but he didn't mind.

 

"Ah, sh-shit, Mettaton, I'm-" Sans' voice broke into a moan.

 

"I know sweetie, you're shivering all over." Ex teased, driving into him harder as Sans began to quake all over, growing more vocal by the second. All at once, Sans felt a massive rush of endorphins and physical pleasure move through him, whining directly into Neo's mouth who was still kissing him and running his fingers along his ribs and clavicle. Sans slowly came down from his orgasm as Mettaton let go of his legs and wrapped his arms around Sans in a hug.

 

"Fuck, guys...that was incredible." Sans mumbled, panting. Mettaton placed a kiss on Sans' head.

 

"So, are you willing to make this a regular thing?" Mettaton purred. Sans jumped up off his lap and started looking for his clothes.

 

He chuckled. "I don't think so, buddy, but-" as he turned around, he felt a soft hand firmly grasp his chin then felt his head being turned to the side. Sans' eyes met with Mettaton's half lidded ones, a cocky grin gracing the robot's lips.

 

"You know you love being our little slut, Sansy." Mettaton purred. Sans shivered as his face blushed bright blue, Mettaton's grasp was released and he stumbled backwards, going back to pulling on his pants.

 

"Could I say no to that face?" Sans chuckled, adjusting the belt around his hips. "But not now, I have work to do, so you two can go home."

 

Mettaton took a showy bow and walked over to Neo, wrapping one arm around the pointer robot's shoulder. "Sure thing, darling. Neo and I have some _frustration_ to work out after all that, don't you agree Neo?"

 

Neo nodded and left a kiss on Mettaton's neck. "We'll see you around, Sans. Thanks for taking a risk with us tonight."

 

Sans smiled warmly as the two left the lab, the room falling silent save for the hum of the computer in front of him. As the skeleton's gaze danced over sums and graphs on the electronic display, and memories of what had just happened ran through his head, Sans assumed he wasn't going to get much work done tonight.

**Author's Note:**

> im so embarassed kill me


End file.
